House conservatives are ready to stop the Senate immigration bill inits tracks with a potent procedural weapon should the contentious measure winpassage in the upper chamber.

The trump card conservatives may hold is a constitutional rule thatrevenue-related bills must originate in the House. The Senate immigrationmeasure requires that illegal immigrants pay back taxes before becomingcitizens, opening the door to a House protest, dubbed a “blue slip” for thecolor of its paper.

House Republicans used the same back-taxes mandate for a blue-slipthreat that derailed last year’s immigration conference. The new Senate billstill must survive two more weeks of voter scrutiny and contentious amendments,but several conservatives already are lying in wait for the Senate to “make thesame mistake twice,” as one House GOP aide put it.